First of all, with a concern for clarity, the disadvantages of prior art are presented here in the particular case of the GSM standard. However, an embodiment of this invention also applies as has been shown to any type of cellular network. Those skilled in the art can easily implement an embodiment of this invention in a network of the UMTS type or other type.
The increasing popularity of the GSM system across the entire world has led the operators to deploy this service not only in the urban regions, but also more and more in rural areas. In this latter type of region, the land infrastructure is often insufficient or poorly adapted to provide the deployment of the network. A radio link system via satellite is then a very good means to extend the GSM service and this type of solution is commonly used today to provide the transmission to remote equipment, such as base stations.
However, satellite resources are costly, and all of the problems with this type of application reside in the techniques for reducing the bandwidth needed for the transmission of data via satellite.
For more clarity, the disadvantages of the prior art are described hereinafter in the specific case of a cellular network infrastructure implemented by the intermediary of a static satellite link or a dynamic satellite link of the DAMA type (for “Demand Assigned Multiple Access”). However, an embodiment of the invention applies more generally to any cellular network implementing a link of the radio with shared resources type.
1. Architecture of GSM
In relation with FIG. 1, the conventional architecture of a cellular network of the GSM type comprises a mobile service switch 10, called MSC (for “Mobile Switching Centre”), a base station controller 11, called BSC (for “Base Station Controller”) and finally one or several base stations 12, called BTS (for “Base Transceiver Station”).
Each BTS provides the GSM radio coverage in one or several cells. By way of example, in relation with FIG. 1, the BTS 121 is controlled by the BSC 11 and covers the geographic cell 13, wherein is located a certain number of users having a Mobile station (MS) for radio communication 14.
More precisely, the MSC controls the configuration of calls for each incoming or outgoing call, and it has the role of an interface with the other telecommunication networks. Each communication goes through the MSC, which controls several BSC.
The BSC is in charge of allocating the radio channels needed for each call. It handles the intercellular transfers between two BTS. A single BSC supports several BTS which provides coverage for a large geographic zone.
Finally, a BTS has for role to carry out the GSM radio transmission with the users of Mobile Stations. The BTS are located in the vicinity of “masts” 122 supporting antennas, and distributed in the geographic space of coverage of the cellular network.
The GSM standard and its evolutions, such as defined by the 3GPP group (for “Third Generation Partnership Project”), make use of voice compression. This compression is carried out by a transcoder also called TC. According to the GSM standard, the TC can be implemented at the MSC site, at the BSC site or at the BTS site. Economic considerations lead to implementing more preferably the TC at the MSC site, so as to reduce transmission costs.
Several types of codecs have been defined by the 3GPP group. The codec GSM FR “full rate” codec operates at a rate of 13 kbit/s. The HR “half rate” and EFR “enhanced full rate” codecs operate at 5.6 kbit/s and 12.2 kbit/s respectively. After transcoding, speech at 64 Kbit/s compressed to 13/12.2 kbit/s (respectively 5.6 kbit/s) is carried to the base station BTS over a time slot at 16 kbit/s (respectively 8 kbit/s). According to the 3GPP TS 08.60 (respectively TS 08.61) specification, the compressed speech is transmitted to the BTS every 20 ms according to the frame format TRAU (for “Transcoder and Adaptation Unit”).
These same principles apply to the AMR (“Adaptive Multi Rate”) full rate FR and reduced rate HR codings.
The TRAU frame carries, in addition to compressed speech data, signalling data of the “control bits” type making it possible to optimise the quality of the communications between the transcoding entity TC and the channel coding/decoding unit CCU (for “Channel Codec Unit”) with the BTS. These control bits make it possible in particular to provide the synchronisation of the data exchanged, to define the type of codings used (FR, EFR, HR or AMR), and also to indicate the discontinuity of the transmission linked to the silence in the speech (DTX).
In such a way as to introduce the implementation of a satellite link within a cellular network, in relation with FIG. 2, the interfaces implemented are now described succinctly and their denomination between the main entities introduced previously.
The PSTN (for “Public Switched Telephone Network”) is denoted as PSTN 22.
The interface between the MSC 10 and a BSC 11 is referred to as interface A.
The interface between a BSC 11 and the BTS 121 is referred to as the interface Abis.
In the case where the TC 21 is implemented at the MSC site 10, the interface between the TC 21 and the BSC 11 is called Ater.
A satellite link can be used within the transmission chain for each of these interfaces. The main problem with inserting a satellite link on one of these interfaces is then to determine how to effectively transmit the necessary data while minimising the radio band needed for the transmission via satellite.
The interface A, used between a MSC and a BSC, is constituted of one or several 2 Mbit/s links (ITU G703/G704 standard). Each 2 Mbit/s link supports 30 uncompressed voice channels—at 64 kbit/s—and one signalling channel SS7. The number of 2 Mbit/s links depends on the sizing of the BSS subsystem. The signalling channel contains messages indicating in particular the traffic needs according to the number of communications.
The interface Abis connects a BSC with a BTS and is constituted of one or several 2 Mbit/s links (ITU G703/G704 standard). It is one of the interfaces which is conventionally implemented with a transmission via satellite.
This interface Abis carries traffic data, such as compressed voice and signalling data.
On the interface Abis, two types of signalling data circulate:                signalling messages exchanged with the BTS, transported in a specific signalling channel, which make it possible to control the BTS equipment itself as well as the mobile station (MS) which are in relation with it. The corresponding messages are specified by the GSM in the TS 08.58 specification.        control “in band” data which is transmitted in the same flow as the traffic data. This data is transmitted within TRAU frames. This data is “control bits”, complementary to the “data bits”, of which the meaning is explained in the TS 08.60/08.61 specifications.        
The signalling data of the first type, constituted of protocol messages, is carried over dedicated time slots, with typically over the interface Abis a rate of 64 kbit/s.
Each 2 Mbit/s link of the interface Abis has 31 time slots (TS) which are allocated to the signalling channels or to the speech channels. According to the typology of the network and coding choices for the speech, a 2 Mbit/s link on the interface Abis can typically be used to support up to ten radio transmission access channels, called TRX (“Transceiver”). Each TRX in turn supports eight GSM channels dedicated to speech at full rate FR or sixteen GSM channels at half rate HR. The corresponding reservation of the speech channels on the interface Abis represents for each TRX an allocation of 2 TS at 64 Kbit/s (8*16 Kbit/s=16*8 Kbit/s=128 Kbit/s).
According to the sizing of the GSM network, the BTS is equipped with a number N of TRXs, which induces a proportional occupation of the number of time slots TS on the interface Abis.
2. Satellite Applications
A conventional GSM network implementing a radio link of the satellite type is described in relation with FIG. 3.
The GSM connecting network then comprises, conventionally, a MSC 30, a BSC 31 as well as a base station BTS 32, providing the communications to users having a mobile terminal 34 and located in the coverage area of the BTS 32.
In addition, a radio link 36 is implemented on the interface Abis, between the BSC 31 and the BTS 32. This radio link 36 is provided by a radio system via satellite containing two antennas 331 and 332 for emitting-receiving on each side of the interface Abis, and a satellite 35.
Note that it is possible in fact to insert a radio link via satellite on each of the interfaces implemented in the GSM system: A, Abis, Ater. But the insertion of such a satellite link on the interface Abis, i.e. between a BSC and BTSs, is very often preferred in order to extend the GSM service to remote geographic locations and of a low density of users with minimal infrastructural costs.
So as to avoid any confusion, it is important to note that in such an implementation, two types of radio systems are implemented, but that they do not have the same role:                The GSM network itself uses a first radio link to communicate, and in particular to carry out the transmission between the BTSs and the users of mobile station.        The satellite system consists of a second radio transmission link. Conventionally, a device called Hub allocates the radio resources needed for the transmission of data by satellite between BSC and BTS.        
In what follows of the description, radio resources are referred to: this denomination thus concerns the radio transmission link via satellite, but it can be extended according to an embodiment of the invention to any other type of radio link with shared resources, as for example links via radio beams (“microwaves”), or systems of the LMDS type (“Local Multipoint Distribution Systems”), or other land transmission systems of the WiFi, WiMAX (for “Wireless Microwave Access”) type, etc.
Very generally, in the case of interest here, the transmission of data between the BTS and the BSC is thus provided by a radio link with shared resources.
Conventionally, in relation with FIG. 3, in the systems of prior art, the satellite link will use a fixed-rate resource for each type of communication (typically 16 kbit/s or 8 kbit/s in GSM), used to carry voice as well as data.
The most common method, with static resources, consists in permanently reserving satellite resources to transport the maximum number of communications likely to be present at the interface Abis. A more effective method with dynamic resources is also sometimes used today, and consists in assigning and releasing the satellite resources as the effective traffic changes between the BSCs and the BTSs. The dynamic allocation of the satellite radio resources, corresponding in fact to frequency and power resources on a satellite transponder, is in general controlled by a central hub, according to a method of the DAMA type.
The DVB-RCS system (for “Digital Video Broadcasting-Return Channel by Satellite”) is an example of a standard utilised to transmit data via satellite. In this type of system, the data is sent in packet mode, in both directions of transmission.
In the “downstream” direction, from the hub to the terminal stations, broadcast channels in the DVB-S (“Digital Video Broadcasting by Satellite”) standard are used and all of the BSC-BTS links are multiplexed in it in a timely manner. In the “upstream” direction, from the terminal stations to the hub, dynamic management of time and frequency resources is performed by the hub, according to the DVB-RSC protocol, in order to:                avoid collisions between the various sources that are sharing the same transponder        guarantee a rate and a transmission delay that is compliant with the quality of communications required.        
An embodiment of this invention applies in particular to configurations using a satellite channel managed in DVB-S/DVB-RCS mode.
3. Techniques for Optimising Radio Resources
A major objective in the implementation of such radio links via satellite in order to realise cellular infrastructure links is to reduce as much as possible the band needed for the various transmissions, so as to reduce costs. To date there are several known techniques.
So as to optimise the allocation of radio resources, techniques have been developed according to which the signalling data contained in the TRAU frames is analysed, thus aiming to avoid the transmission of data during the silences in speech. Likewise, the high rate of inactivity on the signalling channels can be taken into account in order to further reduce the volume of data exchanged. These technologies, sometimes described under the name of “compression” methods, analyse with regards to speech the control bits encapsulated in the frames intended for the TRAU and the BTSs. Such an analysis makes it possible to reduce the rate of the radio channels assigned since it makes use of the fact that speech communications have an effective rate of activity generally in the neighbourhood of 50% (since it is rare to speak and to listen at the same time).
Finally, with regards to the management of radio resources via satellite link, a common technique consists in using a DAMA (for “Demand Assigned Multiple Access”) system. Note moreover that certain satellite systems are of “star” configuration and connect all of the terminal stations with a central hub, while other satellite systems are “meshed” and allow any user to be directly placed into communication, in a single satellite hop, with any other regardless of his position in the network and without necessarily passing through the central hub. The DAMA satellite systems, in star or meshed configuration, optimise the satellite link by dynamically allocating the satellite resources to each active node of the network, according to demand.
For example, a DAMA network can allocate in certain implementations a fixed communication channel to each call, chosen from a reservoir or “pool” of channels. The allocation of resources is then of the on-demand “circuit” type for each terminal. This makes it possible to optimise the downstream and upstream capacity and therefore to increase the number of users on a determined transponder.
Another technique for optimising the allocation of radio resources consists in using a “packet” mode to transmit the data on the shared radio channel and to implement a buffer.
The buffer also makes it possible to store a certain number of speech frames of the TRAU type, or packets containing signalling messages, and to send these frames as “bursts” over a higher rate link. The size of the buffer is therefore typically a number of packets, each one corresponding to a speech frame of 20 ms. Since certain speech frames correspond in fact to silence, and since there is also a high rate of inactivity on the signalling channels, the buffering technique makes it possible to benefit from the activity factor by sending only the significant data packets, which reduces the quantity of radio resources used accordingly. This technique has maximum effectiveness in the downstream connection where a large number of links share a single broadcast channel, but it is more difficult to implement in the upstream direction where each link utilises specific resources to transmit the data to the central hub.
4. Disadvantages of Prior Art
The major disadvantage of the techniques of prior art resides in the fact that they remain costly, due to the implementation of a radio link of the satellite type that is generally over-sized, despite the efforts that have already been made to optimise the allocation of radio resources with the compression of the exchanges of TRAU frames of speech and signalling flows according to the techniques presented previously.
Broadband satellite systems, such as DVB-RCS, have in fact the disadvantage of having been designed substantially for the transfer of Internet data, are therefore poorly optimised for specific applications with real time constraints such as voice.
Indeed, the type of data to be transmitted from or to the Internet can vary significantly, especially in terms of the size of files, but also with regards to the content and the duration of communications. Due to this diversity in the types of data to be transmitted, the current satellite systems have difficulty in guaranteeing a quality of service that is compatible with the real time constraint, and in particular a transfer time that is sufficiently low and constant, except with an increase of the resource allocated in relation to the needs that are strictly necessary:                either statically by permanently allocating speech circuits without taking into account the effective traffic of the cells        or dynamically by allocating for each call an oversized satellite radio channel, without taking into account the detailed knowledge of the exact status of each communication and the actual data transfer needs at each step of the calls.        
It is observed moreover that the current systems via satellite, due to the rather long transfer time of a magnitude of 250 milliseconds for a geostationary-earth-orbiting satellite, do not know how to react quickly to the appearance and the releasing of communications, which has a non-negligible impact on the quality of the communications as perceived by the users, unless once again an over-allocation of radio resources is used.